Many of the theories in population genetics assume that the Mendelian segration ratio of 1:1 is a general rule in many organisms in natural populations. The present study attempts to challenge this basic assumption. The first step in this study is to establish a formal genetic basis for the modes of interaction among elements in the well studied system of Segregation Distorter (SD) in Drosophila melanogaster. It is then planned to analyze the genotypes of the chromosomes in natural populations at Texas with respect to the allelic forms of each of the elements of SD system. The second step is to study segregation frequencies of the chromosomes actually occurring in natural populations. The third step will be to combine the information obtainable from the two previous steps to understand how the segregation frequency of a chromosome pair is regulated in natural populations of this species. Special emphasis will be made on the relationship between the Male Recombination (MR) system and the allelic forms at the elements of SD system. Extention of this project will be made to study the genetic system regulating segregation frequency in the sex chromosome pair (sex ratio regulation) in natural populations. Some related phenomoena such as hybrid dysgenesis, sperm-maternal genotype interaction etc., will also be studied. Results of these studies will require to make re-evaluations of many population genetics predictions on human populations, such as irradiation and chemical hazards through generations.